For large-capacity optical transfer such as 40 Gbit/s and 100 Gbit/s, there are problems such that optical signal-to-noise power limitations need to be overcome and high-density wavelength multiplexing need to be realized. As a technique for overcoming optical signal-to-noise power limitations, utilization of binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) has been known, as an alternative to the conventional On-Off Keying (OOK). Furthermore, to realize high-density wavelength multiplexing, there have been known a method of doubling the number of transfer bits per one symbol by polarization multiplexing that allocates an independent signal to each of two orthogonal polarization components, and a method of increasing the number of transfer bits per one symbol by increasing signal multiplicity such as QPSK and 16 quadrature amplitude modulation (QAM). The QPSK and 16 QAM allocate a signal to an In-phase axis (I-axis) and a quadrature-phase axis (Q-axis) and transfer the signals in an optical transmitter.
Further, a digital coherent method of receiving these optically modulated signals by combining digital signal processing with a synchronous detection method has been attracting attention. In this method, stable separation of multiplexed signals and restoration to the original signals in a receiver can be performed by linear photoelectric conversion using synchronous detection and fixed, semi-fixed, and adaptive linear equalization by the digital signal processing. Therefore, remarkable equalization characteristics and remarkable noise immunity with respect to a linear waveform distortion resulting from wavelength dispersion, polarization-mode dispersion (PMD), or the like occurring in a transfer line can be realized.
Generally, in the digital coherent method, a polarization multiplexing method in which independent signal components (Ex and Ey) are allocated respectively to each of two orthogonal polarization components (an X-polarization component and a Y-polarization component) is used. FIGS. 23 and 24 depict an expression of a time axis of a polarization-multiplexed signal generally used in a conventional polarization multiplexing method. FIG. 23 is an example in which Ex and Ey are bit-synchronized completely on a time axis. FIG. 24 is an example in which Ex and Ey are shifted by a half symbol on a time axis (see, for example, Non Patent Literature 1 and Non Patent Literature 2).